1. Field
The invention relates generally to wireless communications systems, and more specifically to interference cancellation from received signals in a wireless communication system.
2. Background
A typical wireless communication system will include multiple remote stations and multiple base stations. In general, the communication system is bi-directional, with the remote station receiving signals from the base station as well as the remote station transmitting signals to the base station. To facilitate receiving and transmitting signals over the wireless communication channel the remote station includes a receiver, and a transmitter.
A function of the receiver in the remote station is to maximize the amount of desired signal received, while minimizing the amount of any undesired, or interference, signals received. Typically, the desired signals are radio waves arriving from one (1) sector of a three (3) sector base station in close proximity to the remote station. The desired signal carries on it the information that the remote station will decode and use. The undesired, or interference, signals include signals arriving from the other two (2) sectors of the base station “leaking” into the serving sector. In addition, the undesired, or interference signal may be from entirely different base stations that are located near by that are carrying carry information intended for use by other remote stations in the communication system. Signals received by a remote station that are intended for other remote stations interfere with the reception of the desired signal by the remote station making it more difficult to decode the desired signal.
Undesired effects include “interference” and “fading.” Interference refers to all the undesired power that is “pick-up” by the receiver in the remote station. Fading is essentially a kind of self-interference due to the multipath characteristic of a wireless channel. Typically, the desired signal will arrive at the remote station along many paths, due to the desired signal radio waves “bouncing” off buildings, cars, trees, etc. in proximity to the remote station. The multipath signals arrive at the remote station with a random set of phases, such that sometimes the signals add constructively, the signals are in phase, and extra power is received. Other times the signals add destructively, the signals are out of phase, tending to cancel each other out, and lower power is received. Cancellation can be such that for a high-scattering environment, the multipath signal power could drop in strength to 1/100 its average value about 1% of the time. To compensate for the loss in power in the multipath signal, the base station would need to transmit 100 times as much power as if there was no fading in order to keep the receiver working 99% of the time.
There is therefore a need in the art for an effective way to combine signals in a remote station to maximize the usable signal.